1. Field of the Invention
The invention relates generally to vertical-cavity surface emitting lasers being viewed as a light source for use in long wavelength communication, and more particularly to, a method of fabricating long wavelength vertical-cavity surface emitting lasers capable of making the current injection diameter very small and reducing the resistance, through implantation of heavy ions such as silicon (Si) and regrowth of crystals.
2. Description of the Prior Art
Vertical-cavity surface emitting lasers that is grown on an InP substrate as InAlGaAs/InAlAs/InGaAs series is further suited for mass production compared to edge emitting lasers in view of its geometric structure and is thus low in product cost. Therefore, the vertical-cavity surface-emitting laser will be used a lot in the future as a device for a LAN communication network. Especially, the communication distance of vertical-cavity surface emitting lasers having the wavelength of 1.3xcx9c1.55 xcexcm can be extended to several kilometers while that of a 850 nm laser can be extended to several hundred meters. Thus, the vertical-cavity surface emitting lasers are being watched as a light source for a long wavelength communication in a next-generation optical communication network.
There has recently been proposed a structure on which InGaInAs/InAlAs as a material for a long wavelength vertical-cavity surface emitting lasers is grown on an InP substrates[J. Boucart et. al, xe2x80x9cMetamorphic DBR and Tunnel-Junction Injection: A CW RT Monolithic Long-Wavelength VCSELxe2x80x9d, IEEE Journal of Selected Topics I+n Quantum Electronicsxe2x80x9d, Vol. 5, No. 3. p520-529 (1999)].
A current confinement structure being one of the most important factors in fabricating the vertical-cavity surface emitting lasers serves to supply charge carriers to an active region of a given area to control the laser gain area and to thus occur a laser oscillation in the area. For this, conventional vertical surface emitting lasers employ an etched pillar, ion implantation, oxidization, an air gap technique, and the like. However, most of them are based on an AlGaAs material. Thus, there is a difficulty that those techniques are applied to the InAlGaAs/InAlAs long wavelength surface emitting lasers intact.
As the laser pillar can be easily fabricated using a dry ion etching, it is used to manufacture long wavelength vertical-cavity surface emitting lasers. [J. K. Kim, E. Hall, O. Sjolund, G. Almuneau and L. A. Coldren, xe2x80x9cRoom-temperature, electrically-pumped multiple-active-region VCSELs with high differential efficiency at 1.55 xcexcmxe2x80x9d, Vo.35, No. 13, p1084-1085 (1999)]. As the diameter of the pillar is reduced, however, the threshold current is reduced but the resistance is increased in proportion to the square of the diameter. Thus, the entire characteristic of the device is degraded. In addition, as it is etched through the active region, surface recombination is caused on the surface to loss current and to reduce the laser efficiency.
There is a characteristic that the ion dispersion radius is extended in portion to the thickness of a top reflector and a photoresist mask in a proton ion implantation method. Dispersed ions act as a resistance or cause to generate an unstable current injection characteristic. The size of the ion implantation diameter is limited by the thickness of a top reflector. In case of the long wavelength surface emitting lasers, if an InAlAs/InAlGaAs distributed Bragg reflector in the long wavelength laser is used, the size will be 6xcx9c7 xcexcm. It is therefore considered that the minimum diameter that can be possible by ion implantation reaches to 15 xcexcm, considering the thickness of the photoresist. As the thickness of the top reflector is increased along with increase of the wavelength compared to the short wavelength laser, the minimum current injection diameter that can be formed by a proton ion implantation method is limited.
Meanwhile, the oxidization method can form a very efficient current confinement structure. This method, however, could not grow an AlAs layer on the InP substrate in the long wavelength. Therefore, this method is difficult to be realized and its oxidization speed is very low and unstable compared to AlAs even though InAlAs is used.
Finally, the air gap method employs a selective chemical etch property between the AlAs layer and GaAs. This method includes locating the AlAs layer right on the active region and removing it using an HCl solution except for a small diameter. This method also shows a similar characteristic to the oxidization method. In this method, however, AlAs layer is not grown on the InP substrate. Similarly, there is a method by which InP is included within an epitaxial layer and the InP layer is selectively etched. Since InAlGaAs/InAlAs includes Al, it is easily oxidized and attacked by unintentional etchant. Therefore, it is not easy to find a selective etching solution capable of etching InP with a relatively small thickness and a long distance in a horizontal direction.
The present invention contributes to solving the above problems and an object of the present invention is therefore to provide the method of fabricating long wavelength vertical-cavity surface emitting lasers capable of making the current injection diameter very small, through implantation of heavy ions and regrowth of crystal.
In order to accomplish the above goal, a method of fabricating long wavelength vertical-cavity surface emitting lasers according to the present invention, comprises the steps of sequentially growing a bottom distributed Bragg reflector, a laser active region and a heat spreading layer; forming a photoresist mask on the heat spreading layer to define a region for current confinement layer; forming the current confinement layer at a surface portion of the heat spreading layer by ion implantation using the photoresist mask; removing the photoresist mask and sequentially forming an InP layer and a current spreading layer; forming an electrode on the current spreading layer and then stacking a top distributed Bragg reflector; and forming an Au reflector on the surface of the top distributed Bragg reflector.
The ions are silicon (Si) and the ions are implanted with the energy of 50xcx9c500 KeV.